1. Field of the Invention
The present invention relates to a method of estimating a frame error rate of received frames, more particularly to such a method comprising error correction encoding and decoding.
The present invention further relates to a frame error rate estimator, and an apparatus comprising a frame error rate estimator.
Such a method, estimator, and apparatus are particularly useful in CDMA systems where frame error rate estimation is an integrated part of closed-loop power control, including an inner loop and an outer loop.
2. Description of the Related Art
In U.S. Pat. No. 6,154,659, fast forward link power control in a Code Division Multiple Access (CDMA) system is disclosed. In such a system a radiotelephone can assist a base station in the control of the power on the forward link (from the base station to the radiotelephone) by transmitting a power control message to the base station on the reverse link (from the radiotelephone to the base station). The radiotelephone gathers statistics of its error performance and informs the base station via a power control message. In such systems the ratio Eb/N0 or Eb/Nt is a standard quality measurement for digital communications system performance. The ratio expresses the bit-energy-to-noise-density of the received signal. The smaller the required Eb/Nt the more efficient is the system modulation and detection process for a given probability of error. A related metric is the Es/Nt, which is the ratio of symbol-energy-to-noise density of the received signal. The Es/Nt is related to the Eb/Nt by Es/Nt=(Eb/Nt)N where N is the number of bits per symbol. The radiotelephone receives information from the base station. This information is in the form of a stream of data frames. The type and format of this data is well known in the art. The radiotelephone demodulates the information. The Es/Nt is estimated. In an inner loop, the estimated Es/Nt is compared to a target or setpoint Es/Nt. The target Es/Nt is adjusted on a frame by frame basis. In an outer loop, the target is modified in such a way as to maintain a required frame error rate set by network operators. In order to modify the target Es/Nt, the quality of each received frame is determined. If a particular received frame was good, the target is decreased a predetermined amount. If the particular received frame was bad, the target is increased by a predetermined amount. The predetermined amount is set by network operators. Determining the quality of the frames is well known in the art. If the estimated Es/Nt is less than the target Es/Nt, the radiotelephone instructs the base station to power up by a predetermined amount. If the estimated Es/Nt is greater than the target Es/Nt, the radiotelephone instructs the base station to power down by a predetermined amount. The power change commands are transmitted using a reverse power control signaling channel, are punctured in a data stream to the base station, or otherwise transmitted to the base station. Data to be transmitted from the base station to the radiotelephone are encoded using a convolutional encoder or another type of error correcting encoder so as to create redundancy in the original data sequence. Such redundancy improves error correction and signal reconstruction at the receiver. The receiver uses error correction techniques as it decodes the received data to reconstruct the original user data.
3GPP2 Standard xe2x80x9cPhysical Layer Standard for CDMA2000 Spread Spectrum Systemsxe2x80x9d, Release A, publication date Jul. 13, 2001, pp. 2-62 to 2-67, 2-99 to 2-105, 2-125 to 2-127, 2-130 to 2-132, and 2-199 to 2-200 discloses reverse channel types for different radio configurations, including forward error correction types of channels, and convolutional or Turbo encoding on such channels. In addition to data, a frame quality indicator (CRC) is encoded in frames, a frame being a basic timing interval in the system. Such a CRC, Cyclic Redundancy Code, is a class of linear error detecting codes which generate parity check bits. The frame quality indicator is a CRC check that is applied to particular channels in the CDMA system. A mobile station applies an inner and outer power control loop for forward channel power control, and uses a reverse power control sub-channel transmit power control bits to the base station, the value of a power control bit depending on the value of a determined Eb/Nt or Es/Nt. The mobile station demodulation process performs complementary operations to the base station modulation process on the forward CDMA channel, including error correction decoding of received frames.
It is an object of the invention to provide a method of estimating a frame error rate of received frames that takes into account channel conditions of a channel conveying the frames.
It is another object of the invention to provide such a method in which a frame error rate of good frames is determined on the basis of received soft bits and re-encoded hard bits.
It is still another object of the invention to provide such a method of estimating a frame error rate of received frames using at least error correction decoded good frames.
In accordance with the invention, a method of estimating a frame error rate of received frames is provided, the method comprising:
error correction decoding of a received frame;
determining whether the error correction decoded frame is a good frame;
error correction encoding of the good frame; and
estimating the frame error rate on the basis of the error correction encoded good frame and the received frame.
The method is based on the recognition that for frame error rate estimation, in addition to using bad frames, advantageously use can be made of good frames, because, if the frame is correctly received, re-encoded hard bits that occur after error correction decoding and a successive frame quality check are exactly the same as the bits that were transmitted.
In an embodiment an N-tap filter such as a FIR-filter is used to filter frame error rates estimates of good and bad frames, and a noise distribution estimate, that is determined from the error correction encoded good frames and received frames, is used to determine filter coefficients of filtered frame error rate estimates. The noise distribution estimate takes into account effects of channels conditions on a channel conveying the frames, the received frames representing received soft bits.
Because typically there are much more received good frames than bad frames, use of good frames significantly speeds up estimation of the frame error rate and renders a reliable and robust estimation method. Also, because only relatively few frames are needed for obtaining good estimates, the methods are very suitable for fast changing channel conditions.
In an embodiment of the frame error rate estimation method a bit error rate is estimated using the error corrected good frame and the received frame, and the frame error rate is estimated using such a determined bit error rate. Preferably, such a bit error rate (BER) is first determined for a simple receiver such as a BPSK or QPSK receiver, and then, the BER and/or FER for a corresponding forward error correction receiver is determined, preferably through a lookup table that accounts for the coding gain of the forward error correction receiver. Such indirect determination of the BER and/or FER of a FEC receiver is much simpler that determining it directly for a FEC receiver.
In an embodiment of the frame error rate estimation method, the frame error rate is further estimated on the basis of error correction decoded bad frames. Although typically more good than bad frames should be received, such bad frames still contribute to frame error rate estimation.
For determining good frames quality indicators are included in the encoded frames at the transmitting end, such frame quality indictors being CRCs, or any other useful frame quality indicator.
Preferably, error correction encoding and decoding is forward error correction encoding and decoding, applying convolutional encoding and Viterbi decoding, Turbo encoding and decoding, or any other useful encoding or decoding mechanism.
In an embodiment, the frame error rate is estimated for a succession of consecutively received frames, thereby N-tap filtering frame error rate estimates of good and bad frames with diminished filter coefficients for older received frames. The diminishing filter coefficients for older frames effectively serve as a forgetting factor for the information contained in the received frames, good ones and bad ones. I.e., such N-tap filtering, preferably through using an N-tap FIR filter, determines which frames are important in the frame error rate estimation method. Preferably, the window of consecutively received frames is advanced by a predetermined number of frames, whereby the predetermined number of frames and the filter coefficients are determined by the chance in the noise distribution that reflects the change in the channel condition.
In an embodiment, a forward error correction encoder that is already present in a transmit path can also be used to re-encode the decoded hard bits. Through such shared use no additional hardware is needed for such re-encoding.